Interquery Parallelism


Interquery Parallelism
􀂄 Queries/transactions execute in parallel with one another.
􀂄 Increases transaction throughput; used primarily to scale up a
transaction processing system to support a larger number of
transactions per second.
􀂄 Easiest form of parallelism to support, particularly in a sharedmemory
parallel database, because even sequential database
systems support concurrent processing.
􀂄 More complicated to implement on shared-disk or shared-nothing
architectures
􀃌 Locking and logging must be coordinated by passing messages
between processors.
􀃌 Data in a local buffer may have been updated at another processor.
􀃌 Cache-coherency has to be maintained — reads and writes of data
in buffer must find latest version of data.

Cache Coherency Protocol
􀂄 Example of a cache coherency protocol for shared disk systems:
􀃌 Before reading/writing to a page, the page must be locked in
shared/exclusive mode.
􀃌 On locking a page, the page must be read from disk
􀃌 Before unlocking a page, the page must be written to disk if it was
modified.
􀂄 More complex protocols with fewer disk reads/writes exist.
􀂄 Cache coherency protocols for shared-nothing systems are
similar. Each database page is assigned a home processor.
Requests to fetch the page or write it to disk are sent to the home
processor.

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